Resistance welding circuits



June 26, 1945. 5 BURGWlN 2,378,897

RESISTANCE WELDING CIRCUIT Filed Nov. 20, 1943 WITNESSES: f YINVENTORWATT Patented June 26, 1945 RESISTANCE WELDING CIRCUITS Stephen L.Burgwin, Forest Hills, Pa., assignor to Westinghouse ElectricCorporation, East Pittsburgh, Pa., a corporation of PennsylvaniaApplication November 20, 1943, Serial No. 511,044

14 Claims.

This invention relates to a resistance welding control system and hasparticular relation to an electronic system for controlling thedischarge of a power capacitor through the primary winding oi a weldingtransformer.

A system for producing high quality welds known as capacitor dischargewelding has been developed in recent years. A capacitor is prechargedwith a voltage of a magnitude which is preselected in accordance withthe properties of the material to be welded. The charging circuit forthe capacitor is blocked and the capacitor is then discharged rapidlythrough the primary winding of a welding transformer, the secondarywinding of the transformer being connected to supply energy to weldingelectrodes. Discharge of the capacitor through the transformer causessufllcient current to now through the welding electrodes and thematerial clamped therebetween to weld the material. It is customary tooperate such a welding system in a manner to produce a number of weldsin rapid succession.

In apparatus constructed in accordance with the teachings of the priorart; an electric discharge valve of the arc-like type, such as anignitron, is interposed between the primary winding of the transformerand the capacitor to control the instant of discharge of the capacitor.The charging circuit for the capacitor is efiective to charge thecapacitor with one polarity. Consequently, if the capacitor isdischarged through the primary winding of the welding transformer v inthe same direction with'each operation, the transformer becomes highlysaturated after a few successive welds and the efliciency of operationis greatly reduced unless means are provided for resetting the flux ofthe transformer.

Methods of flux resetting now in commercial use employ a separate directcurrent source of potential applied to all or part of the primarywinding to return the flux to its original state after each discharge ofthe capacitor. As so much energy is required to reset the flux within areasonable'time with the welding electrodes closed, considering therapid successive operations desired, this method is believed to bepracticalonly when the welder is used to make a series of spot weldswith the electrodes being opened between successive welds. Then thedirect current potential may be applied to the primaryknown as rollerspot welding or seam welding and in both cases rollers are mounted onthe engaging ends of the welding electrodes and remain in constantengagement with the material to be welded throughout the entire seriesof welds.

In some .welders now in use, relay operated contactors are arranged toreverse the connections of the primary winding of the transformer to thecapacitor between successive welds. Thus the capacitor is dischargedthrough the primary winding in opposite directions for successivewelding operations and saturation of the transformer is avoided eventhough the welding electrodes remain closed. Use of the contactors isobjectionable, however, because of the short life of the contactors andthe time delay imposed between successive welds to permit operation ofthe contactors.

It is, accordingly an object of my invention to provide a new andimproved capacitor discharge welding system in which complete fluxresetting is accomplished quickly and eiiiciently,

Another object of my invention is to provide novel capacitor dischargeweiding apparatus in which complete flux resetting is quicklyaccomplished while the electrodes are closed.

Still another object of my invention is to provide a novel system forsupplying power from a source to a load through a magnetic coretransformer in which saturation of, the transformer is avoided.

A further object of my invention is to provide a novel capacitordischarge welding system in which saturation of the welding transformeris avoided without employing an auxiliary flux resetting potential orreversing contactors.

In accordance with my invention a capacitor is connected in series withthe primary winding of the welding transformer and a source of directcurrent. Current for charging the capacitor then flows from the sourcethrough the primary winding and the capacitor. Circuit means includingelectric valve means is connected in parallel across the seriesconnected capacitor and primary winding. With the capacitor precharged,a damped oscillating current tends to flow through the primary windingand capacitor when the electric valve means is conductive. Means arethen provided to block the flow of charging current after the capacitoris charged and to render the valve means conductive to permit one cycleonly of the oscillating current to flow therethrough. The first pulse ofthe cycle of oscillating current is sumcient to produce a weld but thesecond pulse is smaller and is insumcient to produce a weld.

When the valve means is first rendered conductive. the prechargedcapacitor discharges through the primary winding of the transformer andchanges the flux in one direction. During the discharge, magnetic energyis stored in the welding transformer and the closed secondary circult.After the discharge of the energy originally stored in the capacitor iscompleted, the magnetic energy stored in the transformer and secondarywinding circuit during the discharge is released causing current tocontinue to fiow in the same direction whereby the capacitor is chargedinversely and the fiux is partially reversed. The inverse charge on thecapacitor is immediately discharged back through the primary winding,completing the cycle of oscillating current and further changing theflux in the opposite direction.

As the discharge of the inverse charge on the capacitor is completed,the release of stored magnetic energy causes the capacitor to bepartially recharged with its original polarity. The direct currentcharging circuit is then unblocked and charging of the capacitor throughthe primary winding proceeds. Thus the same amount of charge passesthrough the primary winding in one direction as in the other and theflux is completely reset. A resistor may be connected in shunt acrossthe capacitor to permit a predetermined minimum current to flow throughthe primary winding after the capacitor is charged and thereby toprovide a predetermined flux bias' for the transformer.

The novel features which I consider characteristic of my invention arepointed out with more particularity in the appended claims. Theinvention itself, however, both as to its organization and operation,together with additional objects and advantages thereof, will best beunderstood from the following description of a specific embodiment withreference to the drawing, in which, the single figure is a schematicdiagram of the preferred embodiment of my invention.

A seam welder system is shown in the drawing although my invention mayalso be used in spot welders. The system comprises a, capacitor 3connected in series with the primary winding 5 of -a welding transformer7 and a source of direct current 9. The direct current source 9 includesan auxiliary transformer H having a primary winding l3 energized from asource of alternating current l5 through a circuit breaker i1. Oneterminal of the secondary winding 19 of the transformer II is connectedto the anode 2| of a rectifier tube 23. The other terminal of thesecondary I9 is connected to the anode 25 of a second rectifier tube 21.The rectifier tubes 23 and 21 may be discharge tubes of the arc-liketype such as thyratrons and their cathodes 29 and 3|, respectively, areconnected together-by conductor 33 and to one plate 39 of the capacitor3. The other plate 31 of the capacitor is connected through the primarywinding 9 of the a welding transformer I to the center tap 39 of thesecondary winding l9 of the auxiliary transformer l i. g

The control grids 9| and 93 of rectifier tubes 23 and 21, respectively,are connected together through their respective grid resistors 49 and91. The junction between the grid resistors 99 and 41 is connectedthrough another resistor 99 to the cathodes 29 and 3!. Thus the controlgrids II and 93 are normally at the same potential as their respectivecathodes 29 and 9| so that the tubes 23 and 31 are conductive anddirect. ourrent fiows through the capacitor 3 and the pri-.

mary winding 5 to charge the capacitor with plate 35 positive. Aresistor 5| is connected in shunt across the capacitor 3 to permit apredetermined minimum direct current to flow from the source 9 throughthe primary winding 5 even after the capacitor 3 attains its fullcharge. In this manner a predetermined fiux bias is provided for thewelding transformer which, of course, has a, magnetic core.

A pair of ignitrons 53 and 55 are connected in anti-parallel across theseries connected capacitor 3 and primary winding 5. An electricdischarge valve 51 of the arc-like type, preferably a thyratron andhereafter designated as a firing valve, has its anode 95 connectedthrough a resistor 59 to the anode 6| of the ignitron 53, and itscathode 61 connected to the ignition electrode 63 of the ignitron 53.Another electric discharge valve 99 of the arc-like type, also known asa firing valve has its anode 69 connected through a resistor 10 to theanode 12 of ignitron 55, and its cathode I6 connected to the ignitionelectrode 14 of the ignitron 55. Consequently, when the anode of eitherignitron is positive, the corresponding firing valve may be renderedconductive to supply ignition current through the ignition electrode andthus render the ignitron conductive.

The anode SI of ignitron 53 and consequently the anode 65 of thecorresponding firing valve 51 is connected to the plate 35 of thecapacitor 3 which is originally positive.' However, the oathode 91 ofthe firing valve is connected to the grid 13 through a source of biasingpotential 15 and the secondary winding H or a peaking transformer 19.The peaking transformer 19 is not originally energized and the biasingpotential 15 maintains the grid 13 negative with respect to the cathode61 so that the firing valve 51 associated with ignitron 53 is maintainednon-conductive.

The grid 9| of the firing valve 69 associated with ignitron 59 isconnected through a resistor 93 to the cathode 19 of the firing valve.Thus, whenever the anode I2 of ignitron 55 and consequently the anode 99of the corresponding firing valve 99 becomes positive, the firing valvebecomes conductive to in turn render the ignitron conductive. It is tobe noted that the anode I2 of the ignitron 55 is connected to the plate31 of capacitor 3 which is originally negative.

The secondary winding 95 of the welding transformer H has one terminalconnected to a relatively fixed welding electrode 91 and the otherterminal of' the secondary windin 95 is connected to a movable electrode99. The electrode 99 has a spring biased piston 9| on the end thereofwithin a cylinder 93, to be operated by hydraulic pressure.

To initiate a welding operation, a push button 99 may be depressed toopen an inlet valve 91 and close a pressure release valve 99 in a fluidvalve member I91. Closure of the valve 99 prevents the escape of fluidpressure through a release opening 193. permits the application of fluidpressure from a source not shown through an inlet pip I99, valve 91, andpine I91 to cylinder 99. The pressure on piston 9| in cylinder 93 causesthe electrode 99 to be moved downwardly toward electrode 91 w clamp thematerial I99 to be welded therebetween.

After the electrode 99 is in firm engagement with the material I99, theback pressure through pipe Ill operateson piston, III to close a switchOpening of inlet valve 91 v 31 to supply driving power thereto.

pleted from one side 1' of the alternatingcurrent source I5 through thecircuit breaker I1,

conductor I I9, switch II5, energizing coil I2I of a relay I23, andbreaker IT to the other side I25 of the source. Energization of relayI23 closes itscontactors I21 and I29 completing a circuit from i thesource I5 through a motor I3I As the welder illustrated is to beemployed in seam welding operations wherein a series of individual weldsare to be made Without opening the welding electrodes, the electrodes.and 39 are provided with rollers I33 for engagement'with the materialI09. The material I 09 is to be moved into a new position for each weldby power supplied from motor I 3|. For purposes of illustration, themotor is shown diagrammatically as connected to the roller I33 on thfixed electrode I3I is driven in synchronism with the source I5 and acam member I35 is mounted on a shaft I31 of the motor. The cam memberI35 is arranged to operate a switch I39 which causes a weld to be madein a manner to be hereinafter-explained. The cam member is designed tomaintain the switch I33 closed during a desired welding period and openfor a predetermined time interval between successive welding periods. Ifnecessary, a gear box may be inserted between the motor and the cammember to change the relative speed oi. the cam member. Other suitabletiming, means may, of course, be employed if desired.

When switch I33 is closed, a circuit is completed from one side II1 ofthe source I5 through circuit breaker I1, conductor H3. switch I 39,

contac'tor m of relay in (now closed), energizing coil I43 of anotherrelay I45 and circuit breaker I1 to the other side I25 of the source.Contacior I41 of relay I45, is normally closed to eilect charging of anauxiliary capacitor I49 through a resistor I5I and a full wave rectifierI53 energized from the source l5. When relay I45 is energized, contactorI 41 breaks the charging circuit for auxiliary capacitor I49 and closesthe discharge circuit for the capacitor through the primary I55 f thepeaking transformer 19. The peaking transformer is thus energized andthe impulse supplied from its secondary winding 11 overcomes the biasingpotential supplied from source 15 and renders the firing valve 51associated with'ignition 53 conductive.

At the same time the other contactor I51 of relay I45 closes to impressa direct current biasing potential from an auxiliary source I59 acrossthe resistor 43 in the grid-cathode circuits of rectifier tubes 23 and21. The biasing potential appearing across resistor 59 maintains therectiiier tubes 23 and 21 non-conductive to stop the flow oicharging'current from the source 9 to the main capacitor 3.

When the firing valve 51, associated with ignitron 53, becomesconductive, current begins to flow from the capacitor 3 through theresistor .53; the firing valve 51 and the ignition electrodes 53 torender the ignitron 53 conductive. As soon as the ignitron 53 becomesconductive the potential across the'associated firing valve 51 drops solow that the firing valve becomes non-conductive.

When ignitron 53 is conductive the precharged capacitor 3dischargesthrough the ignitron 53 and the primary winding of the weldingelectrode, causing sufllcient current to be supplied through the weldingelectrodes 83 and 81 and the material III to effect a weld. During thisdischarge of the capacitor 3- magnetic energy is The motor stored in thetransformer 1 and its secondary winding circuit. As the potential acrossthe capacitor 3 drops to zero, this stored energy is released, causingthe current to continue to flow in the same direction. As a result, thecapacitor 3 is charged inversely. When the capacitor 3 attains itsmaximum inverse charge, the current flow drops to zero and the ignitron53 becomes non-conductive. At this time the plate '31 of the capacitor 3i positive and the resultant potential in the discharge circuit is of apolarity that the firing valve associated with ignitron 55 is renderedconductive to in .tum render the ignitron 55 conductive. The inversecharge on the capacitor 3 is then discharged through the primary winding5 of the ignitron55. It is to be noted, however, that because of theenergy spent in welding and the losses in the circuit, the maximuminverse charge on the capacitor 3 is considerably less than the originalcharge so that the discharge of the inverse charge of the capacitorthrough the primary winding does not effect a weld.

Following discharge of the inverse charge on the capacitor through theprimary winding, the capacitor attains a charge of the original polarityby the release of energy stored in the transformer and its secondarywinding circuit during that discharge. ignitron 55 then becomesnonconductive and because of the biasing potential 15,ignitron 53 ismaintained non-conductive. At this time the cam member I 35 opens theswitch I39 to efl'ect deenergization of relay I45. Contactor I51, of therelay I45 opens removing the biasing potential from resistor 49, therebypermitting charging current to again flow from the source 3 through thecapacitor 3 and the primary winding 5. Contactor I41 of the relay I45also closes to permit'recharging of the auxiliary capacitor I43. After apredetermined interval of time during which the material I53 is moved tober I35 recloses-switch I33 and another welding operation is performed.Successive weldsare made in this manner so long as the push remainsdepressed.

As previously indicated a predetermined flux bias is provided for-thetransformer 1 because of the presence of resistor 5I prior to theinitial disbutton 35 charge of the capacitor 3. When the prechargedcapacitor is discharged through the primary winding 5, the flux ischanged in the opposite direction. When the inversely charged capacitoris subsequently discharged through the primary winding, the flux isagain changed in the direc-. tion of the bias but because the inversecharge on the capacitor is'less than the original charge,

the flux is not returned entirely to its original state. Howeventhesubsequent recharging of the capacitor through the primary windingcompletes the resetting of the flux. Thus, the same amount of charge issupplied through the primary winding 5 in one direction as in the otherdirection and saturation of the transformer is prevented. The operationof the welder may consequently be quite rapid with no delays for thepurpose of resetting the flux. Moreover, at the completion of a weld thecapacitor has a small charge thereon of the original polarity so thatlarge transients at the initiation of recharging are avoided andconservation of power is eifected.

Although I have shown and described a preferred embodiment of myinventiomI am aware 4 asvaee'l that many modifications thereof arepossible. an

invention, therefore, is to be limited only by the spirit of theinvention and the scope of the prior art.

' I claim as my invention:

1. For use in supplying current to a load through a'magnetic coretransformer having a primary and a secondary winding with said secondarywinding connected in circuit with said load, the combination comprisinga capacitor connected in a series circuit with said primary winding,means adapted to supply direct current through said series circuit tocharge said capacitor, circuit means including normally non-conductiveelectric'valve means connected to form a discharge circuit across saidcapacitor and primary winding whereby an oscillating current tends tofiow through said discharge circuit when said valve means is conductive,and means for rendering said valve means conductive to permit only onecycle of said oscillating current to fiow therethrough.

2. For use in supplying current to a load through a magnetic coretransformer having a primary and a secondary winding with .saidsecondary winding being connected in circuit with said load, thecombination comprising a capacitor connected in a series circuit withsaid primary winding, means adapted to supply direct current throughsaid series circuit to charge said capacitor, circuit means includingelectric discharge valve means of the arc-like type connected acrosssaid capacitor and primary winding whereby an oscillating current tendsto flow through said capacitor and primary winding when said valve meansis conductive, and means for rendering said valve means conductive topermit only one cycle of said oscillating current to fiow therethrough.

3. For use in supplying current to a load through a magnetic coretransformer having a primary and a secondary winding with said secondarywinding being connected in circuit with said load, the combinationcomprising means adapted to function as a source of direct current, acapacitor connected in series with said source means and said primarywinding whereby said capacitor is precharged, a resistor connected inshunt across said capacitor to permit a predetermined minimum current tofiow through said primar winding after said capacitor is charged toprovide a predetermined flux bias for said transformer, circuit meansincluding electric discharge valve means connected across saidcapacitorand primary winding whereby an oscillating current tends to fiow throughsaid capacitor and primary winding when said valve means is con-vductive, and means for rendering said valve means conductive to permitonly one cycle of said oscillating current to fiow therethrough.

4. For use in supplying current to a load through a magnetic coretransformer having a primary and a secondary winding with said secondarywinding being connected in circuit with said load, the combinationcomprising a capacitor connected in a series circuit with said primarywinding, means adapted to supply direct current through said seriescircuit to charge said capacitor, a first electric discharge valve ofthe arc-like type connected in parallel with said series-connectedcapacitor and primary-winding, means for rendering said first valveconductive after said capacitor is charged, said first valve beingcapable of conducting current in a direction to effect discharge of saidcapacitor through said primary winding whereby said capacitor issubsequently charged inversely, a second QM? tric discharge valve of thearc-like type connected in anti-parallel with said first valve, andmeans operable when said first valve becomes non-conductive followingsaid discharge for rendering said second valve conductive to effectdischarge of the inversely charged capacitor through said primarywinding.

5. For use in supplying current to a load through a magnetic coretransformer having a primary and a secondary winding with said secondarywinding being connected in circuit with said load, the combinationcomprising means adapted to function as a source of direct current, acapacitor connected in series with said primary winding and said sourcemeans whereby said capacitor is charged, a resistor connected in shuntacross said capacitor to permit a predetermined minimum current to fiowthrough said primary winding after said capacitor is charged to providea predetermined fiux bias for said transformer, a first electricdischarge valve of the arc-like type connected in parallel with saidseries-connected capacitor and primary winding, means for rendering saidfirst valve conductive after said capacitor is charged, said first valvebeing capable of conducting current in a direction to effect dischargeof said capacitor through said primary winding whereby said capacitor issubsequently charged inversely, a second electric discharge valve of thearc-like type connected in anti-parallel with said first valve, andmeans operable when said first valve becomes non-conductive followingsaid discharge for rendering said second valve conductive toefiectdischarge.

of the inversely charged capacitor through said primary winding.

6. For use in supplying current to a load through a magnetic coretransformer having a primary and a secondary winding with the secondarywinding being connected in circuit with said load, the combinationcomprising means adapted to function as a source of direct current. acapacitor connected in series with said source means and said primarywinding whereby said capacitor is charged, circuit means includingelectric valve mean connected to form a discharge circuit across saidcapacitor and primary winding whereby an oscillating current tends tofiow through said discharge circuit when said valve means is conductive,and timing means operable through said discharge circuit, said timingmeans being operable repeatedly at timed intervals of sufiicient lengthto permit charging of said onpacitor between successive periods.

'7. Apparatus for use in welding a material with equipment includingwelding electrodes in en v tl-gement with said material and a magneticcore valve conductive after said capacitor is charged, said first valvebeing capable of conducting current in a direction to effect a firstdischarge of said capacitor through said primary winding whereby saidcapacitor is subsequently charged inversely to a second voltage less inmagnitude than said first voltage, a second electric discharge valve ofthe arc-like type connected in anti-parallel with said first valve, andmeans operable when said first valve becomes non-conductive followingsaid discharge for rendering said second valve conductive to effect asecond discharge of the inversely charged capacitor through said primarywinding, the magnitudes of said first and second voltages being suchthat said first discharge is sufiicient and said second discharge isinsufilcient to produce a Weld.

8. Apparatus for use in welding a material with equipment includingwelding electrodes in engagement with said material and a magnetic coretransformer having a primary and a secondary winding with said secondarywinding being connected in circuit with said electrodes, comprisingmeans adapted to function as a source of direct current, a capacitorconnected in series with said primary winding and said source meanswhereby said capacitor is charged to a first voltage, a resistorconnected in shunt across said capacitor to permit a predeterminedminimum current to flow through said primary winding after saidcapacitor is charged to provide a predetermined flux bias for saidtransformer, a first electric discharge valve of the arc-like typeconnected in parallel with said series-connected capacitor and primarywinding, means for rendering said first valve conductive after saidcapacitor is charged, said first valve being capable of conductingcurrent in a direction to effect a first discharge of said capacitorthrough said primary winding whereby said capacitor is subsequentlycharged inversely to a second voltage les in magnitude than said firstvoltage, a second electric discharge valve of the arc-like typeconnected in anti-parallel with said first valve, and means operablewhen said first valve becomes non-conductive following said dischargefor rendering said second valve conductive to effect a second dischargeof the inversely charged capacitor through said primary winding, themagnitudes of said first and second voltages being such that said firstdischarge is sufiicient and said second discharge is insufilcient toproduce a weld.

9. Apparatus for use in welding a material with equipment includingwelding electrodes in engagement with said material and a magnetic coretransformer having a primary and a secondary winding with said secondarywinding being connected in circuit with said electrodes, comprisingmeans adapted to function as a source of direct current, a capacitorconnected in series with said source means and said primary windingwhereby said capacitor is charged, circuit means including electricvalve means connected to form a discharge circuit across said capacitorand primary winding whereby a damped oscillating-current tends to flowthrough said discharge circuit when said valve means is conductive, andtiming means operable to block the fiow of current from said sourcemeans to said series-connected capacitor and primary winding and saidvalve means for a. predetermined time period and to render said valvemeans conductive during said period to permit only one cycle of saidoscillating current to flow through said discharge circuit, said timingmeans being operable repeatedly at timed intervals of sumcient length topermit charging of said capacitor between successive periods, the firstpulse of said one cycle of oscillating current being sufflcient toproduce a weld and the second pulse being insufficient to produce aweld.

10. Apparatus for use in welding a material with equipment includingwelding electrodes in engagement with said material and a magnetic coretransformer having a primary and a secondary winding with said secondarywinding being connected in circuit with said electrodes, comprisingmeans adapted to function as a source of direct current, a capacitorconnected in series with said source means and said primary windingwhereby said capacitor is charged, a resistor connected in shuntacrosssaid capacitor to permit a predetermined minimum current to flow throughsaid primary winding after said capacitor is charged to provide apredetermined fiux bias for said transformer, circuit means includingelectric valve means connected to form a discharge circuit across saidcapacitor and primary winding whereby a damped oscillating current tendsto flow through said discharge circuit when said valve means isconductive, and timing means operable to block the fiow of current fromsaid source means to said series-connected capacitor and primary windingand said discharge circuit for a predetermined time period and to rendersaid valve means conductive during said period to permitonly one cycleof said oscillating cur-- pulse being insufficient to produce a weld.

11. For use in supplying current to a load through a magnetic coretransformer having a primary and a secondary winding with said secondarywinding being connected in circuit with said load, the combinationcomprising a capacitor connected in a series circuit with said primarywinding, means adapted to supply direct current through said seriescircuit to charge said capacitor, a first electric discharge valve ofthe arc-like type connected in parallel with said series-connectedcapacitor and primary winding. timing means operable after saidcapacitor is charged to block the flow of current from said currentsupply means to said series circuit and said first valve for apredetermined-time period and to render said first valve conductiveduring said period, said first valve being capable of conducting currentin a direction to effect discharge of said capacitor through saidprimary winding whereby said capacitor is subsequently chargedinversely, a second electric discharge valve of the arc-like typeconnected in anti-parallel with said first valve, and means operablewhen said first valve becomes non-conductive following said dischargefor rendering said second valve conductive to efiect discharge of theinversely charged capacitor through said primary winding, said timingmeans being operable repeatedly at timed intervals of sufiicient lengthto permit charging V of said capacitor between successive periods. 12.Apparatus for use in welding a, material with equipment including amagnetic core transformer having a primary and a secondary winding withthe secondary winding being connected in circuit with welding electrodesengaging the material, comprising .a capacitor connected in a seriescircuit with said primary winding. means adapted to supply directcurrent through said series circuit to charge said capacitor to apredetermined voltage, circuit means including normally-non-conductiveelectric valve means connected to form a discharge circuit across saidcapacitor and primary winding whereby a damped oscillating current tendsto flow through said discharge circuit when said valve means isconductive, and means for rendering said valve means conductive topermit only one cycle of said osan oscillating current tends to flowtherethrough when said valve means becomes conductive, and means forrendering said valve means conductive to permit a single cycle only ofoscillating current to flow through said load.

14. For use in supplying current to a load through a magnetic coretransformer having a primary and a secondary winding with said secondarywinding being connected in circuit with said load, the combinationcomprising a capacitor connected in a series circuit with said primarywinding, means adapted to supply direct current through said seriescircuit to charge said capacitor, a first electric discharge valve ofthe arelike type connected in parallel with said seriesconnectedcapacitor and primary winding, means for rendering said first valveconductive after said capacitor is charged, said first valve beingcapable of conducting current in a direction to effect discharge of saidcapacitor through said primary winding whereby said capacitor issubsequently charged inversely, a second electric discharge valve of thearc-like type connected in anti-parallel with said first valve, andmeans responsive to a change in the polarity of the potential acrosssaid second valve when said capacitor is charged inversely and operablewhen said first valve becomes non-conductive following said dischargefor rendering said second valve conductive to effect discharge of theinversely charged capacitor through said primary winding.

STEPHEN L. BURGWIN.

